A linear tape is typically moved longitudinally across a read/write head so that the head can read and/or write data in parallel longitudinal data tracks along the tape. Servo bands are provided parallel to the data tracks, and at least one servo head reads the servo bands. A servo system responds to the lateral position of the servo read head to operate an actuator to move the read/write head transversely with respect to the tape to follow a particular path at the same lateral position with respect to the servo band, such that the read/write head follows the same data tracks.
Timing based servo arrangements are employed with respect to linear tape media, for example, in magnetic tape systems, to provide track following capability. Recorded servo patterns comprise dual magnetic transitions called “servo stripes” arranged as pairs recorded at more than one azimuthal orientation across the tape media. The technology is discussed in the incorporated U.S. Pat. No. 5,689,384. Reading a servo pattern by a servo read head yields a sequence of pulses called “dibits”, where each dibit corresponds to the transitions at the edges of a servo stripe written on the magnetic medium. The timing between dibits that correspond to any pair of servo stripes with different azimuthal orientation thus varies continuously as the servo read head is moved in the lateral direction across the servo band. The pattern is read by a servo read head whose width is small compared to the servo band pattern, and servo read head position is derived from the relative timing of pulses generated by the servo read head reading the servo pattern as the tape is moved in the longitudinal direction. Position sensing with this system is achieved by deriving a ratio of two servo pattern intervals, one pattern interval comprising the timing between a transition of each of the dibits corresponding to a pair of servo stripes having different azimuthal orientation, and the other pattern interval comprising the timing between a transition of each of the dibits corresponding to a pair of servo stripes having the same azimuthal orientation. Thus, the position sensing is ratio dependent and is insensitive to tape speed.
The linear tape is typically subject to transverse movement as it is being moved in the longitudinal direction across the read/write head, and track following is employed to allow the read/write head to follow any transverse movement of the tape. The sensed position is employed for track following in which an actuator moves the read/write head transversely of the linear tape to attain a desired lateral position of the servo read head along the servo band and to trace a path following the servo band and thereby follow data tracks with data heads that are typically laterally offset from the servo read head. The sensed position is compared to the desired lateral position of the servo read head with respect to the servo band, and the difference is used to derive a position error signal and operate the actuator.
The primary need of the track following control system is to have a high sampling rate servo feedback signal to support a high bandwidth track following servo system. A high sampling rate provides up to date, accurate information of the read/write head. The timing based servo system measures the time between servo transitions of different bursts of servo transitions, for example, arranged in the form of subframes and frames, where a subframe comprises two bursts, and a frame comprises two subframes. The bursts of a subframe are separated by a first pattern interval or gap, and the subframes of a frame are separated by a second pattern interval or gap. The current timing based servo systems employ the time between servo transitions of two bursts of a subframe and the time between servo transitions of two subframes, such that the sampling rate encompasses a full frame. Thus, should tape velocity slow, the sampling rate may become too slow to sustain a high bandwidth track following system.